Zosterophyll
| Zosterophyllopsida Temporal range: Ludlow to Devonian |
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| Zosterophyllum species fossils | |
| Scientific classification | |
| Kingdom: | Plantae |
| Division: | Tracheophytes |
| Subdivision: | Lycophytina |
| Class: | †Zosterophyllopsida |
The Zosterophylls are a group of extinct plants. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosterophyllophyta and the class Zosterophyllopsida. They were among the first vascular plants in the fossil record, and had a world-wide distribution. They were probably stem-group lycophytes, forming a sister group to the ancestors of the living lycophytes. By the late Silurian (late Ludlovian, about 420 million years ago) a diverse assemblage of species existed, examples of which have been found fossilised in what is now Bathurst Island in Arctic Canada.
The stems of zosterophylls were either smooth or covered with small spines known as enations, branched dichotomously, and grew at the ends by unrolling, a process known as circinate vernation. The stems had a central vascular column in which the protoxylem was exarch, and the metaxylem developed centripetally. The sporangia were kidney-shaped (reniform), with conspicuous lateral dehiscence and were borne laterally in a fertile zone towards the tips of the branches.
The zosterophylls were named after the aquatic flowering plant Zostera from a mistaken belief that the two groups were related. David P. Penhallow's generic description of the type genus Zosterophyllum refers to "Aquatic plants with creeping stems, from which arise narrow dichotomous branches and narrow linear leaves of the aspect of Zostera."Zosterophyllum rhenanum was reconstructed as aquatic, the lack of stomata on the lower axes giving support to this interpretation. However, current opinion is that the Zosterophylls were terrestrial plants, and Penhallow's "linear leaves" are interpreted as the aerial stems of the plant that had become flattened during fossilization.
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